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Steganography - Hiding messages in the Noise of a Picture

, 12 Jul 2004
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Hiding all kinds of data in the pixels of bitmaps.

Sample Image - steganodotnet.jpg

Introduction

This article describes how any data can be hidden in the noise pixels of an image.

Background

There is only one safe place for private data and messages: the place where nobody looks for it. A file encrypted with algorithms like PGP are not readable, but everybody knows that there is something hidden. Wouldn't it be nice, if everyone could open your encrypted files, and see noisy photos of some old friends instead of your private data? They surely wouldn't look for pieces of encrypted messages in the pixel values.

Using the code

To hide a message, open a bitmap file, then enter a password or select a key file. The key file can be any file, another bitmap for example. This password or key will be treated as a stream of bytes specifying the space between two changed pixels. I don't recommend text files, because they may result in a quite regular noise pattern. The longer your key file or password is, the less regular the noise will appear.

Next step, enter the secret message or choose a file, and click the Hide button. The application writes the length of the message in bytes into the first pixel. After that it reads a byte from the message, reads another byte from the key, and calculates the coordinates of the pixel to use for the message-byte. It increments or resets the color component index, to switch between the R, G and B component. Then it replaces the R, G or B component of the pixel (according to the color component index) with the message-byte, and repeats the procedure with the next byte of the message. At last, the new bitmap is displayed. Save the bitmap by clicking the Save button. If the grayscale flag is set, all components of the color are changed. Grayscale noise is less visible in most images.

To extract a hidden message from a bitmap, open the bitmap file and specify the password or key you used when hiding the message. Then choose a file to store the extracted message in (or leave the field blank, if you only want to view hidden Unicode text), and click the Extract button. The application steps through the pattern specified by the key and extracts the bytes from the pixels. At last, it stores the extracted stream in the file and tries to display the message. Don't bother about the character chaos, if your message is not a Unicode text. The data in the file will be all right. This works with every kind of data, you can even hide a small bitmap inside a larger bitmap. If you are really paranoid, you can encrypt your files with PGP or GnuPG before hiding them in bitmaps.

How it works

To see how the application works, you should view the source. I've commented every block, because otherwise I'd have to post nearly all the code here to explain it.

Here is a summary about hiding:

And about extracting:

The process starts with writing the length of the entire message into the first pixel. We'll need this value before extracting the message later on.

messageLength = (Int32)messageStream.Length;


//do some length conflict checking here
//...
String colorValue = messageLength.ToString("x");
colorValue = UnTrimColorString(colorValue, 6);
int red = Int16.Parse(colorValue.Substring(0,2), NumberStyles.HexNumber);
int green = Int16.Parse(colorValue.Substring(2,2), NumberStyles.HexNumber);
int blue = Int16.Parse(colorValue.Substring(4,2), NumberStyles.HexNumber);
pixelColor = Color.FromArgb(red, green, blue);
bitmap.SetPixel(0,0, pixelColor);

Then it reads a byte from the key stream to calculate the next position:

//start with second pixel
Point pixelPosition = new Point(1,0);
//Loop over the message
for(int messageIndex=0; messageIndex<messageLength; messageIndex++){
 //repeat the key, if it is shorter than the message
 if(keyStream.Position == keyStream.Length){
  keyStream.Seek(0, SeekOrigin.Begin);
 }

 //Get the next pixel-count from the key, use "1" if it's 0
 currentKeyByte = (byte)keyStream.ReadByte();
 currentStepWidth = (currentKeyByte==0) ? (byte)1 : currentKeyByte;

 //jump to reverse-read position and read from the end of the stream
 keyPosition = keyStream.Position;
 keyStream.Seek(keyPosition, SeekOrigin.End);
 currentReverseKeyByte = (byte)keyStream.ReadByte();
 //jump back to normal read position
 keyStream.Seek(keyPosition, SeekOrigin.Begin);

 //Perform line breaks, if current step is wider than the image
 while(currentStepWidth > bitmapWidth){
  currentStepWidth -= bitmapWidth;
  pixelPosition.Y++;
 }

 //Move X-position
 if((bitmapWidth - pixelPosition.X) < currentStepWidth){
  pixelPosition.X = currentStepWidth - (bitmapWidth - pixelPosition.X); 
  pixelPosition.Y++;
 }else{

  pixelPosition.X += currentStepWidth;
 }

Now, get the pixel and put the message-byte into one color component (or all components, if the grayscale flag is set):

 //Get color of the "clean" pixel
 pixelColor = bitmap.GetPixel(pixelPosition.X, pixelPosition.Y);

 //To add a bit of confusion, xor the byte with 
 //a byte read from the keyStream
 int currentByte = messageStream.ReadByte() ^ currentReverseKeyByte;

 if(useGrayscale){
  pixelColor = Color.FromArgb(currentByte, currentByte, currentByte);
 }else{
  //Change one component of the color to the message-byte
  SetColorComponent(ref pixelColor, currentColorComponent, currentByte);
  //Rotate color components
  currentColorComponent = 
   (currentColorComponent==2) ? 0 : (currentColorComponent+1);
 }
} //end of for - proceed to next byte

If the method runs in extraction mode, it reads the message's length and the color components instead of setting it. Here is how to get the length from the first pixel:

pixelColor = bitmap.GetPixel(0,0);
//UnTrimColorString fill the String with '0'-chars 
//to match the specified length
String colorString = UnTrimColorString(pixelColor.R.ToString("x"), 2)
 + UnTrimColorString(pixelColor.G.ToString("x"), 2)
 + UnTrimColorString(pixelColor.B.ToString("x"), 2);

 messageLength = Int32.Parse(colorString, NumberStyles.HexNumber);
 messageStream = new MemoryStream(messageLength);

The pixel coordinates are calculated the same way as described above. Then the hidden byte is extracted from the color value:

 //Get color of the modified pixel
 pixelColor = bitmap.GetPixel(pixelPosition.X, pixelPosition.Y);
 //Extract the hidden message-byte from the color
 byte foundByte = (byte)(currentReverseKeyByte ^ 
   GetColorComponent(pixelColor, currentColorComponent));
 messageStream.WriteByte(foundByte);
 //Rotate color components
 currentColorComponent = 
   (currentColorComponent==2) ? 0 : (currentColorComponent+1);
} //end of for - proceed to next byte

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Corinna John
Software Developer
Germany Germany
Corinna lives in Hannover/Germany (CeBIT City) and works as a Delphi developer, though her favorite language is C#.

Comments and Discussions

 
GeneralRe: Check out a commercial version PinmemberCorinna John16-Sep-03 8:25 
GeneralRe: Check out a commercial version PinmemberSoliant7-Feb-06 8:40 
GeneralNeed a little help with the SaveAs-problem PinmemberCorinna John14-Sep-03 8:36 
GeneralRe: Need a little help with the SaveAs-problem PineditorHeath Stewart14-Sep-03 9:12 
GeneralRe: Need a little help with the SaveAs-problem PinmemberCorinna John15-Sep-03 3:00 
GeneralRe: Need a little help with the SaveAs-problem PineditorHeath Stewart15-Sep-03 3:26 
GeneralRe: Need a little help with the SaveAs-problem PinmemberCorinna John16-Sep-03 8:19 
GeneralCouldn't make it work Pinmemberadrian738-Sep-03 20:41 
It's the second time when I try this sample, first time there were some compiling error, now I receceive an exception message
"Set pixel is not supported for images with indexed pixel formats."
Anyway, I can only select bitmap files and I think all bitmaps have indexed pixel format...

GeneralRe: Couldn't make it work PinsussCorinna9-Sep-03 22:35 
GeneralRe: Couldn't make it work PinsussAnonymous10-Sep-03 1:42 
GeneralRe: Couldn't make it work PinmemberCorinna John11-Sep-03 7:46 
Questionbinary version? Pinmemberagenthypo6-Sep-03 20:18 
AnswerRe: binary version? PinmemberCorinna John6-Sep-03 22:22 
Generalother language Pinmemberidothat6-Sep-03 9:06 
GeneralRe: other language PinmemberCorinna John6-Sep-03 22:29 
GeneralRe: other language Pinmemberidothat7-Sep-03 9:57 
GeneralWouldn't it be less obvious... Pinmemberpeterchen31-Aug-03 11:48 
GeneralRe: Wouldn't it be less obvious... PinmemberCorinna John31-Aug-03 19:39 
GeneralRe: Wouldn't it be less obvious... PinmemberJörgen Sigvardsson21-Sep-03 6:06 
GeneralRe: Wouldn't it be less obvious... PinmemberCorinna John21-Sep-03 8:15 
GeneralRe: Wouldn't it be less obvious... PinmemberCorinna John5-Oct-03 19:48 
GeneralRe: Wouldn't it be less obvious... Pinmemberpeterchen5-Oct-03 20:11 
GeneralSecurity enhancements PinmemberAdam Nelson31-Aug-03 6:35 
GeneralRe: Security enhancements PinmemberCorinna John31-Aug-03 6:44 
GeneralRe: Security enhancements PinsussKoka14-Sep-03 7:16 

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